Endohedral metallofullerenes are materials which have metal atoms inside their fullerene cages. With electron transfer from the metal atoms to the fullerenes, the endohedral metallofullerenes change in electric properties and electron transport properties of the fullerenes. Therefore, it is believed that the endohedral metallofullerenes can be used in a wide variety of fields such as a medical field and an electronic equipment field. Studies are underway to use, for example, a gadolinium-encapsulated endohedral metallofullerene (Gd@C82) in contrast dyes for nuclear magnetic resonance imaging (MRI), a lutetium-encapsulated endohedral metallofullerene (Lu3N@C80) in beta-ray emitting therapeutic agents, a lithium-encapsulated endohedral metallofullerene (Li@C60) in n-type semiconductors, and a terbium-encapsulated endohedral metallofullerene (Tb@C82) in single-molecule nanodevices.
A method for producing an endohedral metallofullerene comprises forming endohedral metallofullerene-containing soot by arc discharge or the like using, as a raw material, a composite rod prepared by mixing graphite powder and metal oxide powder, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-159,514 (PTL 1).
The endohedral metallofullerene-containing soot contains about 0.1% of an endohedral metallofullerene. Accordingly, the endohedral metallofullerene has been isolated from the endohedral metallofullerene-containing soot by high-performance liquid chromatography (HPLC).
In the method for separating the endohedral metallofullerene by the HPLC, however, about half of the endohedral metallofullerene is adsorbed by a stationary phase in a column. Therefore, loss of the endohedral metallofullerene is large. If all the endohedral metallofullerene adsorbed by the stationary phase is to be recovered, a developing solvent must be passed through the column a lot of times. Accordingly, efficiency in recovering the endohedral metallofullerene is low. Life cycle of the column is relatively short and the column needs to be regularly replaced with a new one.
Moreover, the HPLC needs a large amount of a developing solvent, and it takes a long time to separate an endohedral metallofullerene by the HPLC. For example, separation of 10 mg of an endohedral metallofullerene by the HPLC takes a duration of 1 to 2 months or more. Endohedral metallofullerenes which encapsulate short-half-life elements or unstable substances need to be separated especially rapidly. Besides, the HPLC for separating an endohedral metallofullerene requires a special kind of column, hardware, etc.
Under these circumstances, Steven Stevenson et al (NPL 1) succeeded in extracting Sc3N@C78 and Sc4O2@C80 by causing a fullerene solution which contains Sc3N@C78, Sc4O2@C80, etc. to selectively react with AlCl3 or FeCl3 and precipitate. However, the method of Steven Stevenson et al does not attain a sufficiently high recovery yield of endohedral metallofullerenes.
Furthermore, Imre Busci et al (NPL 2) reported that when fullerenes C60 and C70 are reacted with AlCl3, highly reactive C70 forms a complex and C60 is separated. However, it takes 2 to 6 days to cause a reaction by the method of Imre Busci et al. Therefore, even if the method of Imre Busci et al is applied to separation of an endohedral metallofullerene, it cannot be expected to separate the endohedral metallofullerene in a short time.
On the other hand, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-504,700 (PTL 2) discloses a method comprising forming stable fullerene cations in a solvent from one fullerene group selected from a first fullerene group and a second fullerene group in a fullerene mixture and thereby separating the one fullerene group from the other fullerene group.